Systematic analyses of transcripts in early embryogenesis by the group of Dr. M. Ko (q.v. in these Annual Reports) have uncovered a relative dearth of X-linked genes. Nevertheless, 15 X-linked genes showed a high level of expression. Two of these have been chosen for special study.One, isolated as a cDNA from the ectoplacental cone (that is, the first tissue that gives rise to placenta) turns out to be a previously unisolated member of the Bex family, Bex-3. The transcribed sequence is 789 bp in length and encodes a protein of 124 residues with a CAAX motif at its C-terminal end. It appears in a single cDNA and Northern species of 1.1 kb. Expression declines after the fetal period, but adult mouse and human show sustained expression in several organs, including brain, testis and ovary. The putative Bex-3 amino acid sequence shows very high homology to a partially characterized human cDNA clone called pHGR74, which is associated with spontaneous ovarian granulosa cell carcinoma. In vitro-expressed Bex3 is attached to the plasma membrane, and we are examining the possibility that it may be involved in signal transduction during mouse development.The second gene is an ortholog of an X-linked human gene, ZNF127-Xp, which had been partially recovered in cDNA form and shown to escape X-inactivation. The mouse gene is expressed at increasingly high levels as development proceeds, with mRNA species of 3.4, 1.9 and 0.8 kb detected in several tissues; in adult mouse and human, the largest cDNA species shows especially marked expression in testis. Sequencing of the full-length 1.9 kb cDNA species recombinant GST-RING protein constructs revealed a putative protein of 515 amino acids, containing 4 C3H motifs, 1C2H2CH motif and near the carboxy terminus, a RING-finger motif(C3HC4). Cell expression of recombinant GST-RING protein constructs show that the C-terminal portion is associated with cytoskeletal or nuclear matrix fractions, and we are continuing to examine specific attachment partners as a start toward understanding its function.